Type-II InAs/GaSb Strained Layer Superlattice (T2SL) is an emerging technology for infrared detection. They are being seen as a threat to 50 years old incumbent technology based on Mercury-Cadmium-Telluride (MCT) system. T2SL have been theoretically predicted to outperform MCTs. This dissertation has been aimed at improving the physical understanding and detector performance of T2SL detectors.
This work has been focused on the bandstructure simulation, molecular beam epitaxy (MBE) growth optimization, physical understating, and heterojunction barrier engineering of T2SL, in midwave infrared (MWIR, 3-5m) and longwave infrared (LWIR, 8-12m) regimes. The bandstructure of InAs/GaSb/AlSb superlattices has been simulated in this work using empirical pseudopotential method. The simulation results have been used extensively in the later part of this work for designing heterojunction T2SL photodetectors. The interfaces between the individual layers (InAs and GaSb) in T2SL have been optimized for improving the detector performance. Performance of different interface structures were evaluated using variable temperature photoluminescence response, and with the use of detector performance parameters such as responsivity, detectivity and dark current. It was concluded that supporting “InSb” type bonds at both the interfaces resulted in the best performance. This was achieved by using Sb2 soak time on “InAs on GaSb” interface and growing thin InSb layer on “GaSb on InAs” interface. This also helps in strain balancing the system. This interface scheme has been subsequently used in the high performance LWIR pBiBn devices as well as the HOT MWIR cascade devices, giving lattice matched T2SL in these structures. Physical properties of T2SL system are not very well understood, for example the nature of minibands. Polarization sensitive photocurrent spectroscopy measurements were carried out on MWIR and LWIR T2SL material, and experimental results were correlated with the theoretical simulations to unambiguously establish the ordering of valence minibands in T2SL system. It was found that the ordering of minibands with the increase in energy in the valence band is HH1, LH1, LH2 and HH2.
The performance of InAs/GaSb T2SL photodiodes was improved by barrier engineered devices. Device architecture, called pBiBn, was proposed in this work, and was realized for MWIR and LWIR detectors. It uses unipolar electron and hole current blocking layers to reduce the various components of the dark currents. LWIR devices optimized for this device architecture demonstrated the performance close to the state of the art in T2SL technology with the dark current density of 1.42x10-5A/cm2, with cutoff wavelength on 10m at 76K. MWIR pBiBn device also demonstrated high operating temperature (HOT). The capability of multicolor detection using InAs/GaSb/AlSb system was demonstrated in this work by realizing two-color and three color detectors, using three contact architecture, for SWIR/MWIR/LWIR detection.
An interband cascade detector based on InAs/GaSb T2SL system was implemented in this work for HOT application. It consisted of absorber, tunneling and transport regions and all the regions were based on InAS/GaSb/AlSb system. The MWIR (c=5m, T=77K) cascade detector demonstrated operation up to 420K, which is the highest reported operating temperature in MWIR regime, while the LWIR (c=9.6m, T=80K) device operated till 200K with an optimal bias range of a few mV. This work has demonstrated the flexibility of InAs/GaSb/AlSb superlattice system by heterojunction device designs and their successful implementation.